1. Field of the Invention
This invention relates to electronic circuits, and more particularly, to voltage controlled oscillators (VCOs).
2. Description of the Related Art
Phase-locked loops (PLLs) are commonly used for the generation of oscillating signals in a wide variety of electronic circuits. For example, PLLs may be used in digital circuits for clock generation and reproduction, in radio transmitter and receiver circuits for generation of signals at radio frequencies, and so forth.
A typical PLL includes a phase detector coupled to receive a reference signal, a low pass filter coupled to the output of the phase detector, and a voltage controlled oscillator (VCO). The output of the VCO may provide the output signal of the PLL, and may also be fed back to an input of the phase detector via a feedback loop. In some instances, the output of the VCO may be divided before being provided to the phase detector. The phase detector is configured to compare the reference signal and the signal received from the feedback loop and adjust its output voltage accordingly.
One type of VCO commonly used in PLLs is based on a ring oscillator having an odd number of inverters (or inverting amplifiers) coupled together to form a ring (some embodiments may be implemented using an even number of inverters). The frequency of a signal output by such a ring oscillator may be varied based on a voltage provided thereto. More particularly, when the voltage is increased, the frequency of the ring oscillator may also increase, while a decrease in voltage may result in a reduction of the frequency. In manufacturing circuits that utilize a ring oscillator-based VCO, it may be necessary to control for variations in process, voltage, and temperature to ensure satisfactory operation with respect to such factors as jitter, phase error, and so on.